The present invention relates generally to digital aperture synthesized radiometers, and more particularly to an aperture synthesized radiometer employing digital beamforming including interferometry and which is capable of imaging microwave energy emitted or reflected by earth surface features.
Microwave radiometers are passive remote sensors which can be designed to detect, measure and display the microwave energy emitted by earth surface features and the atmosphere. Such systems, operating at L-band, can be used for soil moisture measurements from a space platform orbiting the earth.
In order to remotely measure soil moisture, three primary steps are involved. These include identifying moisture within the field of view of the measuring system's sensors, measuring the detected moisture level, and displaying the measured levels. Substantially all objects in the natural environment emit electromagnetic energy in the microwave region. The amount of energy emitted by an object is related to its physical temperature and surface emissivity, and it normally varies with frequency.
Moist soil naturally radiates over a large bandwidth in the microwave region, including L-band. Therefore, to measure the moisture content of soil, it is advantageous to tune the remote sensor to the L-band frequency region. Once the soil moisture is identified and measured, it must be imaged. This is done by constructing a ground map of the soil moisture levels. In order to do this, two dimensional ground resolution is required.
One way to achieve soil moisture imaging is to employ a system described in U.S. Pat. No. 4,724,439, entitled "Microwave Radiometer Using Fanbeam Inversion," issued to Carl Wiley and Charles Edlesohn, and assigned to the assignee of the present invention. In this patent, a microwave radiometer is disclosed which reconstructs images using fanbeam inversion. A scanning radiometer utilizes a fanbeam antenna which is mechanically scanned across the scene so that the antenna output is a projection of the scene taken along the direction of the scan. The scanning motion is provided for by a motor coupled to the antenna and controlled by a computer. An inverse Radon transform is employed in the computer to reconstruct the scene.
However, although the above scanning radiometer system performs admirably, the development of digital computers and signal processors provides a means to accomplish the microwave imaging without the use of moving parts or motors, or the like.